A significant portion of the world's methanol is produced by the catalytic reaction of synthesis gas obtained by reforming hydrocarbons. The synthesis gas may be produced in a steam reformer, an autothermal reformer, or a partial oxidation reformer containing hydrogen, carbon monoxide, and carbon dioxide.
The majority of hydrogen is produced from a synthesis gas produced by the mentioned reforming technologies. For hydrogen production the hydrogen content in the syngas shall be as high as possible whereas for methanol production a suitable synthesis gas composition may be characterized by a hydrogen-carbon oxide molar ratio defined as:
            [              H        2            ]        -          [              CO        2            ]                  [      CO      ]        +          [              CO        2            ]      where [H2], [CO], and [CO2] are the mole fractions of the respective components in the synthesis gas.
The methanol production typically takes place at location where the hydrocarbon feedstock e.g. natural gas is available at low cost e.g. Trinidad. The methanol is then stored and transported on a global basis to its consumers. Compared to methanol hydrogen as a gaseous product cannot be transported economically over long distances and therefore is typically produced at the location where the H2 product is also consumed e.g. refineries or chemical complexes. However certain refineries or chemical complexes also have a demand for methanol. For those instances a co-production of methanol inside the hydrogen production plant can reduce the production cost, logistics cost and also reduce or eliminated emissions of criteria pollutant generated during the transportation of methanol e.g. shipping via tankers.
FIG. 1 illustrates a typical synthesis gas (syngas) plant for hydrogen production as known to the art. A light hydrocarbon, natural gas in this example, is fed into a reformer. A steam methane reformer is indicated in FIG. 1, but the above discussed processes apply equally wed, depending on the type of feedstock, desired ratio of carbon monoxide, carbon dioxide and hydrogen. Depending on the available natural gas supply pressure, a natural gas feed compressor may be needed. As the syngas is generated at a very high temperature, this gas stream may be cooled in a process gas boiler, thereby producing steam which may be useful elsewhere and thus improving the thermal efficiency of the facility.
If additional hydrogen is desired, a water gas shift reactor may be utilized. Any additional useful heat in the shifted syngas stream may then be extracted in a syngas waste heat recovery unit. As high purity hydrogen is often the desired product from such a system, a hydrogen separation device, a pressure swing adsorption unit in FIG. 1, may be used to separate the hydrogen for export.
FIG. 2 illustrates a combined hydrogen and methanol production facility as known to the art. (see U.S. Pat. No. 6,706,770 for example) A light hydrocarbon, natural gas in this example, is fed into a reformer. A steam methane reformer is indicated in FIG. 2, but the above discussed processes apply equally well, depending on the type of fuel, desired ratio of carbon dioxide and hydrogen, etc. Depending on the available natural gas supply pressure, a feed compressor may be needed. As the syngas is generated at a very high temperature, this gas stream may be cooled in a process gas boiler, thereby producing steam which may be useful elsewhere and thus improving the thermal efficiency of the facility.
In the process scheme of FIG. 2, the cooled syngas is split into a first stream that is combined with process steam and enters the shift reactor (as discussed above). Then into a waste heat recovery unit, and then a hydrogen separation device, such as a pressure swing adsorption unit, to produce hydrogen for downstream use. The cooled syngas is split into a second stream that enters a second waste heat recovery unit, then is compressed and then introduced into a methanol reactor, thus producing a crude methanol stream for use downstream.
In order to utilize the synthesis gas most efficiently in the above reactions, stoichiometric amounts of hydrogen and carbon oxides are preferred. Synthesis gas with a suitable stoichiometric composition for methanol production has a value of the hydrogen-carbon oxide molar ratio of 2.0-2.4. Methanol is produced by reacting the synthesis gas catalytically in a pressurized reactor to yield methanol and unreacted synthesis gas, the methanol is condensed and separated from the unreacted synthesis gas, and a portion of the unreacted synthesis gas is recycled to the reactor feed to increase overall conversion. A certain percentage of the unreacted synthesis gas must be purged from the methanol reactor loop so that components who may be present the synthesis gas but not participating in the methanol synthesis e.g. N2 and CH4, Ar do not build up in the reactor feed gas.
Synthesis gas produced by steam reforming of light hydrocarbons generally contains excess hydrogen when used for methanol production. Thus while purging inert components out of the methanol synthesis loop a significant amount of unreacted hydrogen must be withdrawn and may be used as waste fuel. This purge gas also contains valuable carbon oxides, which become unavailable for conversion to methanol, and this loss adversely affects methanol production economics.
Several approaches to minimize the amount of purge gas or to valorize the purge gas differently have been utilized in commercial methanol production. In one approach, imported carbon dioxide is mixed with either the synthesis gas feed to the methanol reactor or the feed hydrocarbon to the steam reforming step. This gives a methanol reactor feed gas that is closer to the preferred stoichiometric composition, but is possible only when a source of carbon dioxide is readily available. In another approach, unreacted synthesis gas is separated by various methods into a stream enriched in carbon oxides and a stream enriched in hydrogen, the carbon oxide-rich stream is recycled to the reformer or the methanol reactor, and the hydrogen-enriched stream is used for fuel. Membrane systems, absorption processes, and pressure swing adsorption have been used to effect separation of the unreacted synthesis gas.
An alternative approach is to generate the synthesis gas by methods other than steam reforming wherein these methods produce a synthesis gas closer to the preferred hydrogen-carbon oxide ratio for methanol production. Known methods to generate the preferred synthesis gas composition include the partial oxidation, autothermal reforming, and a two-stage process comprising steam reforming followed by oxygen secondary reforming. These methods all require a supply of oxygen, however, and the capital costs are higher than for simple steam reforming.
There is clearly a need in the industry for a more energy efficient and cost effective system for the co-production of hydrogen and methanol.